A light receiver has the function of generating an electrical signal from incident received light. The detection sensitivity of simple photodiodes is not sufficient in a number of application cases. In an avalanche photodiode (APD), the incident light triggers a controlled avalanche effect. The charge carriers generated by incident photons are thus multiplied and a photocurrent is produced that is proportional to the received light intensity, but that is in this respect substantially larger than with a simple PIN diode. In the so-called Geiger mode, the avalanche photodiode is biased above the breakdown voltage such that a single charge carrier released by a single photon can already trigger an avalanche that then recruits all the available charger carriers due to the high field strength. The avalanche photodiode thus, like the eponymous Geiger counter, counts individual events. Geiger-mode avalanche photodiodes are also called SPADs (single photon avalanche diodes).
The high radiation sensitivity of SPADs is utilized in a number of applications. They include medical engineering with CT, MRT or blood analyses, optical metrology including spectroscopy, distance measurement and three-dimensional imaging, radiation detection in nuclear physics or uses in telescopes for astrophysics.
Geiger-mode APDs or SPADs are therefore very fast, highly sensitive photodiodes based on semiconductors. A disadvantage of the high sensitivity is that not only a useful light photon, but also a weak interference event due to external light, optical crosstalk or dark noise can trigger the avalanche effect. This interference event then contributes to the measurement result with the same relatively strong signal like the received useful light and can also not be distinguished therefrom out of the signal. The avalanche photodiode subsequently remains insensitive for a dead time of approximately 5 to 100 ns and is down for further measurements for this time. It is therefore customary to interconnect a plurality of SPADs and to evaluate them statistically.
To be able to actually use the electrical signal, it must be read out of or tapped from the SPAD detector element. Conventional signal tapping circuits are, however, slow and can therefore not cope with high frequency signals. In addition, the signals of the SPADs are simply combined in many known applications. It is, however, not possible therewith to associate the signals with the triggering SPADs, for example in a matrix image sensor.
WO 2011/117309 A2 proposes providing a third electrode at the SPAD detector, in addition to the anode and the cathode for the application of the bias, via which third electrode the Geiger current can be capacitively decoupled. It should thereby be prevented that the reading out is delayed by switching elements of the bias. The document does not, however, look at the actual signal tapping.